1. Field of Invention
The present invention relates to a system and method for recording and reproducing holographic storage, and more particularly, to a system and method which has an optical servo.
2. Related Art
Today, the storage capacity of commercial blue-ray disks rarely exceeds 100 GBytes, and various approaches to achieve super-high storage capacity have been widely researched and developed. The holographic disk is the most important approach of all. The holographic storage technology has been developed for long time. However, many factors make it difficult to be applied to commercial optical storage products. For example, in the early days holographic experiments had to employ a large laser source with high power of several hundreds of milliwatts, and a complex optical servomechanism, and a large and heavy anti-vibration system. Meanwhile, photo-refractive crystals serving as holographic storage media are too expensive, so that the development of holographic storage is limited. Presently, miniaturized high-power laser sources, high-photosensibility materials, and miniaturized optical servomechanism have been developed significantly. Also, inspired by the track-at-once optical disks, such as CD-R and DVD-R, it has gradually become a common view that holographic storage is not necessarily to be rewritable. Many organic materials that are cheap and have high photosensibility are used as the data layer of holographic storage media, such as photopolymer. When the photopolymer suffers a light irradiation with high intensity, the molecular arrangement of the photopolymer is changed to change the optical characteristics of the photopolymer, such that the photopolymer can be used to record and reproduce three-dimensional holographic interferograms. The miniaturized optical servomechanism derived from the CD or DVD players is also vital to the application of holographic storage technology.
Holographic storage technology, for example, Joint International Symposium on Optical Memory and Optical Data Storage 2005 (ISOM/ODS 2005), Hawaii, US, paper ThE2 discloses a transmissive holographic storage media for data storage. The approach of the paper include a two-dimensional image sensor and a spatial light modulator disposed on the same side of a holographic storage media, and a reflecting mirror set disposed on the other side of the holographic storage media. When reproducing data, a reproducing reference beam is reflected by the reflecting mirror set, and travels in a direction which is opposite the direction of the recording reference beam. Thus, the reproducing reference beam is projected to the holographic storage media. However, to a continuously moving holographic storage media, it is very difficult to adjust the position and the direction of the reflecting mirror set on time.
Other related art is disclosed in U.S. Pat. No. 6,721,076 and U.S. Pat. No. 6,909,529. They put forward a mechanism for a reflective holographic storage media, but they also failed to provide optical servomechanism for a continuously moving holographic storage media.
US Patent Publication No. 20040212859 discloses a recording method for transmissive holographic storage media. The method includes a two-dimensional image sensor and a spatial light modulator disposed on different sides of the holographic storage media. A servo beam is projected to the servo tracks of the holographic storage media via the objective lens. But the holographic interferogram is recorded only on a single layer, so the capacity of storage is limited.